*:• 


The  Color-Sensitiveness  of 
Photo-Electric  Cells 


ELEANOR  FRANCES  SEILER 

A.B.,  Uni-versily  of  Den-ver,  igiy 
yi.M.,  Unix'ersity  of  Deni'tr,  ig/g 
A.  M. , Umi'tnity  of  lllinoh,  ig/6 


THESIS 

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OF  DOCTOR  OF  PHILOSOPHY  IN  PHYSICS  IN  THE  GRADUATE 
SCHOOL  OF  THE  UNIVERSITY  OF  ILLINOIS 
>92^- 


r 


UNIVERSITY  OF  ILLINOIS 


THE  GRADUATE  SCHOOL 


-192- 


1 HEREBY  RECOMMEND  THAT  THE  THESIS  PREPARED  UNDER  MY 
^SUPERVISION  BY_  ELEANUR  FRAXCES-SEXLEB 


ENTITLED_  THR.  COLOR  SECSITIUEURSS  OF  P-] 


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THE  DEGREE  OF 


DOCTOR  ■■OF,  -PHILOSOPHY  lU  PHYSICg- 


In  Charge  of  Thesis 


Head  of  Department 


Recommendation  concurred  in* 

[^.  Q , , 

/ 


Committee 


on 


Final  Examination* 


•Rei]uired  for  doctor’s  degree  hut  not  for  master's 


Digitized  by  the  Internet  Archive 
in  2015 


https://archive.org/details/colorsensitiveneOOseil 


The  Color-Sensitiveness  of 
Photo-Electric  Cells 


BV 


ELEANOR  FRANCES  SEILER 


A.  B. , Un  'vvenity  of  Denver,  /gij 
A.M.,  University  of  Denver,  igi4 
A.M.,  University  of  Illinois,  igib 


THESIS 

SUBMITTED  IN  PARTIAL  FULFILLMENT  OF  THE  REQUIREMENTS  FOR  THE  DEGREE 
OF  DOCTOR  OF  PHILOSOPHY  IN  PHYSICS  IN  THE  GRADUATE 
SCHOOL  OF  THE  UNIVERSITY  OF  ILLINOIS 


192^ 


Reprinted  from  tlie  Ahtuopii ysicai,  Joi  k.\al, 
Vol.  LM,  nuiiilicr  3,  October  1920,  page  129. 


Se.A 


THE 

ASTROPHYSICAL  JOURNAL 

AN  INTERNATIONAL  REVIEW  OF  SPECTROSCOPY 
AND  ASTRONOMICAL  PHYSICS 


VOLUME  Lll  OCTOBER  1920  NUMBERS 


COLOR-SEXSmVEXESS  OF  PHOTO-ELECTRIC  CELLS 

By  ELEANOR  FRANCES  SEILER 
ABSTRACT 

Color-sensitivcntss  curves  of  thirty  photo-electric  cells,  including  all  the  alkali  metals 
and  hydrides  of  Xa,  K,  Rb,  and  Cs. — As  great  care  was  used  in  mounting  the  apparatus 
rigidly,  in  calibrating  the  thermo-couple  and  wave-length  scale,  and  in  maintaining 
a constant  source  of  light,  the  curves  were  accurately  determined  and  this  enabled 
the  wave-lengths  of  maximum  sensitiveness  to  be  located  within  about  i nfi.  It  was 
found  that  as  the  atomic  weight  of  the  alkali  metal  increases,  the  maximum  sensi- 
tiveness decreases,  the  resonance  peak  becomes  broader,  and  Xmax  shifts  toward 
the  red.  The  author  suggests  that  these  changes  may  be  associated  with  the  increase 
in  atomic  volume.  For  glass  cells  filled  with  argon  at  low  pressure,  Xmax  was  405, 
419,  440,  473,  and  539AIAI  for  Li,  Na,  K,  Rb,  and  Cs  respectively,  while  for  the  cor- 
responding hydrides  Nall,  KH,  Rbll,  and  CsII,  the  values  were  larger;  427,  456, 
481,  and  S40/i|i  respectively.  With  neon  instead  of  argon,  in  the  case  of  Nall  and 
KH,  Xmax  was  about  20 mm  shorter.  And  while,  in  the  case  of  KII,  pyrex  cells  gave 
the  same  curve  as  glass  ones,  quartz  cells  showed  a longer  Xmax  for  both  KII  and 
Rbll,  contrary  to  what  one  would  expect  from  differences  in  absoq>tion.  Since  the 
effects  of  gas  and  of  cell  wall  were  not  eliminated  and  the  normal  and  selective  photo- 
electric effects  were  not  difTerentiated,  the  curves  are  characteristic  of  the  particular 
cells  rather  than  of  the  various  metals  alone;  nevertheless  it  is  of  interest  to  note 
that  the  products  of  Xmax  with  (i)  resonance  potential,  (2)  ionization  potential, 
and  (3)  absolute  melting  temperature  of  the  corresponding  metals  are  each  roughly 
constant.  The  hydride  cells  were  somewhat  more  sensitive  than  the  corresponding 
metal  cells. 

Fatigue  tests  of  two  photo-electric  cells.  The  effect  of  illuminating  a K cell  for 
525  hours  was  to  increase  its  sensitiveness  by  about  70  per  cent,  whereas  a KII  cell 
remained  constant  in  sensitiveness  cluring  1000  hours’  illumination. 

Preparation  of  a lithium  photo-electric  cell.  It  was  found  possible  to  dissolve 
lithium  in  aethylamine  if  absolutely  dry  and  if  a trace  of  ammonia  was  present. 
uniformly  distributed  layer  of  lithium  was  obtained  by  evaporating  the  solution. 

Method  of  preparing  pure  aethylamine  is  described. 

I 2Q 


130 


ELEANOR  FRANCES  SEILER 


I.  INTRODUCTION 

In  1918  T.  Shinomiya'  carried  out  an  experimental  determi- 
nation of  the  maximum  wave-length  color-sensitiveness  point  (Xmax) 
of  photo-electric  cells  of  the  pure  alkali  metals  sodium,  potassium, 
and  rubidium,  and  of  the  colloidal  modification  of  the  alkali  hy- 
drides of  these  same  metals.  He  found  that  the  Xmax  was  not  the 
same  for  the  pure  metal  and  the  hydride,  but  that  a shift  occurred, 
the  nature  of  which  was  not  fully  determined. 

The  object  of  this  investigation  was  threefold: 

1.  To  obtain  more  consistent  results  on  the  shifting  phenome- 
non and  to  secure  conclusive  experimental  proof  of  its  direction  and 
amount  by  the  use  of  many  more  cells  and  the  extension  of  the 
work  to  include  all  the  alkali  metals.  No  satisfactory  theory  has 
been  advanced  to  explain  the  cause  of  this  shifting  of  the  wave- 
length of  maximum  sensitiveness.  However,  the  results  of  this 
investigation  prove  the  existence  of  such  a phenomenon,  and 
furthermore  they  show  the  shift  to  be  consistently  always  in  the 
same  direction,  namely,  toward  longer  wave-lengths  when  the  alkali 
metals  were  sensitized  by  means  of  hydrogen. 

2.  To  complete  the  study  of  the  color-sensitiveness  for  the 
whole  group  of  alkali  metals.  When  the  photo-electric  cell  is  to 
be  used  in  making  photometric  measurements,  it  is  necessary  to  have 
a complete  knowledge  of  its  curve  of  sensitiveness.  In  scientific 
literature  it  is  found  that  the  curves  given  contain  only  a few  points, 
which  hardly  suffice  to  determine  the  point  of  maximum  sensitive- 
ness or  the  general  nature  of  the  curves  with  much  accuracy.  The 
ideal  photo-electric  cell  would  have  the  same  color-sensitiveness 
as  the  human  eye. 

3.  To  furnish  data  for  theoretical  purposes  in  the  application 
of  the  quantum  theory  to  chemistry.  No  general  agreement  has 
yet  been  reached  regarding  the  relation  of  the  selective  to  the  nor- 
mal photo-electric  effect.  Wiedemann^  found  that  hydrogen  was 
essential  for  the  selective  effect  of  potassium.  The  results  of  this 
experiment  gave  a decided  selective  effect  in  cells  where  the  potas- 
sium was  repeatedly  distilled  in  vacuo.  According  to  Xlillikan 

Astro  physical  Journal,  49,  303,  1919. 

^ Berichle  der  deutschen  physikalischen  Gesellschaft,  18,  333,  1916, 


COLOR-SENSITIVENESS  OF  PHOTO-ELECTRIC  CELLS  131 


and  Souder*  there  is  an  “essential  identity”  of  the  two  effects. 
This  investigation  was  not  carried  on  with  the  purpose  of  getting 
either  effect  to  the  elimination  of  the  other.  Primary  interest  was 
centered  in  the  photometric  properties  of  the  cell  and  not  of  the 
alkali  metals  as  such.  Apparently  there  is  considerable  difference 
in  sensitiveness  when  the  absorption  of  light  through  the  glass  is 
taken  into  account  and  when  gas  is  present  in  the  cells. 

II.  THE  .APPARATUS  AND  METHOD 
A.  THE  CALIBRATIONS 

In  order  to  measure  the  energy  of  illumination  throughout  the 
visible  spectrum  and  the  corresponding  wave-lengths  of  light,  it 
was  necessary  to  make  four  calibrations,  about  every  month  during 
the  course  of  the  investigation.  We  will  call  these  calibrations: 
(i)  thermo-couple,  (2)  scale-energy,  (3)  spectrometer,  and  (4)  scale- 
wave-length. 

I.  Thermo-couple.  A thermo-couple,  furnished  by  W.  W. 
Coblentz  from  the  Bureau  of  Standards,  was  calibrated  by  means 
of  the  light  from  a Hefner  lamp.  Chemically  pure  amyl  acetate 
was  put  into  the  lamp,  which  was  placed  exactly  one  meter  from  the 
thermo-couple  and  in  the  same  horizontal  plane  with  it.  Both  were 
placed  in  a large  two-compartment  box,  which  was  completely 
blackened.  The  partition  contained  a shutter,  by  means  of  which 
the  light  could  be  allowed  to  fall  u])on  the  thermo-couple,  and  which 
acted  as  a partial  diaphragm.  A hooded  chimney  was  placed 
above  the  lamp  to  take  care  of  the  heat  from  the  llame.  A small 
glass  window,  with  a horizontal  line  on  it,  was  set  in  the  box  directly 
behind  the  image  of  the  llame,  so  that  the  flame  could  be  accurately 
adjusted  to  the  proper  height  and  observed  from  the  outside, 
d'he  box  being  tight  and  free  from  air  currents,  the  llame  soon  came 
to  an  equilibrium  condition.  'Fwo  line  wires  led  from  the  thermo- 
couple to  an  insulating  plug  in  the  back  of  the  box.  d'here  they 
were  soldered  to  heavy  double  silk-wound  copper  wires,  which  led 
through  the  plug  down  a heavy-walled  wooden  tube  to  a galva- 
nometer. 'Fhis  tube  served  the  purpose  of  keeping  the  temperature 

' Proceedings  of  the  National  Academy  of  Science,  2,  19,  1916. 


132 


ELEANOR  FRANCES  SEILER 


constant  and  of  preventing  air  currents.  The  lead  wires  in  the 
tube  were  insulated  by  passing  down  the  center  over  strips  of  hard 
rubber.  The  galvanometer  was  a Leeds  and  Northrop  high 
sensitivity  type,  whose  figure  of  merit  was  2.7X10”®.  It  was 
inclosed  in  a box  about  which  tin  foil  was  wrapped  for  earthing. 

These  precautions  made  the  thermo-galvanometer  deflections 
exceedingly  steady,  and  a reading  could  be  repeated  time  and  time 
again  with  not  as  much  as  i per  cent  of  variation.  The  mean  of 
twenty  readings  gave  a deflection  of  1 2 1 mm.  The  energy  radiating 
from  a Hefner  lamp,  according  to  K.  Angstrom,''  is  1089  ergs  per 
sec.  per  sq.  cm  at  a distance  of  one  meter.  Hence  a deflection  of 
I mm  corresponded  to  a flow  of  energy  of  9 ergs  per  sec.  per  sq.  cm. 
The  slit  of  the  thermo-couple  was  a trifle  narrower  than  the  slit  L 
(Fig.  2)  admitting  light  to  the  photo-electric  cell,  so  that  the  former 
was  always  covered  by  light.  The  area  of  L was  0.2842  sq.  cm, 
and  hence  the  energy  passing  through  that  slit  per  mm  of  deflection 
of  the  galvanometer  of  the  thermo-couple  was  9-0.2842  or  2.558 
ergs. 

2.  Scale-energy.  The  thermo-couple  was  placed  with  its  slit 
directly  behind  L and  held  there  rigidly.  It  w'as  connected  to  the 
same  galvanometer  through  another  tube,  which  was  completely 
wrapped  with  tin  foil  and  earthed. 

By  means  of  a fine  screw  R (Fig.  2),  the  thermo-couple  was  moved 
along  the  millimeter  scale  S through  the  entire  spectrum,  and  the 
mean  of  three  readings  taken  as  the  deflection  at  each  point.  Then 
by  plotting  a curve  with  scale-readings  as  abscissae  and  energy  in 
ergs  as  ordinates,  the  energy  incident  upon  every  point  in  the 
spectrum  was  exactly  determined. 

3.  Spectrometer.  Since  the  Hilger  constant-deviation  spectrom- 
eter was  to  be  used  in  determining  the  wave-length  incident  at  every 
corresponding  point  on  the  scale  S,  it,  too,  had  to  be  cahbrated. 
This  was  done  by  means  of  a Plucker  discharge  tube.  The  prism 
of  the  spectrometer  was  adjusted  so  that  the  corrections  were  very 
slight. 

4.  Scale-wave-length.  The  wave-lengths  corresponding  to  the 
points  along  the  scale  were  then  obtained  with  this  instrument. 

‘W.  W.  Coblentz,  Bulletin  of  Bureau  of  Standards,  11,  05,  igi4. 


COLOR-SENSITIVENESS  OF  PHOTO-ELECTRIC  CELLS  133 


The  lines  seen  in  the  spectrometer  were  almost  as  fine  as  those  seen 
when  the  discharge  tube  was  used,  showing  that  the  light  was  very 
nearly  monochromatic.  They  became  somewhat  broader  toward 
the  red  end  of  the  spectrum,  but  this  was  unavoidable  and,  as  the 
experiment  did  not  require  great  accuracy  in  that  region,  accuracy 
was  sacrificed  to  get  better  definition  in  the  other  end  of  the  spec- 
trum. By  plotting  a curve  with  wave-lengths  as  abscissae  and 
scale-readings  as  ordinates,  the  wave-length  corresponding  to  any 
position  on  the  scale  was  obtained. 

B.  THE  EXPERIMENT 

I.  The  optical  system.  The  optical  system  used  in  this  inves- 
tigation was  an  improvement  over  that  used  by  T.  Shinomiya  in 
that,  by  the  use  of  two  large  prisms  and  a condensing  lens  L4  (Fig.  i). 


greater  dispersion  and  better  monochromatism  were  obtained. 
As  the  sensitiveness  of  the  photo-electric  cell  changes  in  a con- 
tinuous way  along  the  spectrum,  the  very  slight  deviation  from  the 
ideal  condition  of  absolute  monochromatism  had  little  efiect  on 
the  final  curves. 

'I'he  set-up  is  shown  diagrammatically  in  Figure  i.  The  source 
of  light,  F,  was  a specially  constructed  tungsten  17-ampere  nitrogen- 
filled  lamp  made  by  the  Nela  Research  Laboratory.  The  bright 
filament  was  a straight,  narrow  fiat  strip  which  acted  as  a line 
source.  A Weston  direct-current  ammeter.  A,  measured  the  cur- 
rent through  the  lamp,  d'his  had  to  be  kept  very  constant  at  17 
amperes,  for  the  slightest  variation  in  the  intensity  of  the  light 
made  itself  felt  in  both  the  thermo  and  photo-electric  galvanometers, 
d’he  current  was  adjusted  by  a tin  resistance.  Hi,  and  a fine  adjust- 
ment sliding  resistance  Rj  in  parallel.  Two  sets  of  storage  bat- 
teries connected  in  parallel  served  as  the  source  of  current.  They 
were  kept  well  charged  and  the  resultant  current  was  steady. 


134 


ELEANOR  FRANCES  SEILER 


The  light  was  condensed  by  lenses  Li  and  and  brought  to  a 
focus  on  the  slit  S,  which  was  o . 7 mm  in  width.  It  was  then 
rendered  parallel  by  the  lens  The  diaphragm  was  adjusted 
so  that  the  front  face  of  the  prism  Pi  was  just  covered  by  light. 
The  position  of  minimum  deviation  for  green-blue  light  was  chosen 
for  both  prisms.  After  passing  through  the  prisms  the  various 


colors  of  the  spectrum  were  brought  to  a focus  and  made  pure  by 
the  lens  P4.  The  spectrum,  10  cm  in  length,  was  beautifully 
distinct  and  bright.  The  reflected  and  stray  light  was  eliminated 
by  the  diaphragms  D^,  D2,  and  D^,  and  further  by  placing  the  entire 
optical  system  in  a box  made  of  thin  compo-board,  completely 
blackened  on  the  inside.  The  slit  L (Fig.  2)  was  accurately  located 


COLOR-SEX  SIT  IV EN ESS  OF  PHOTO-ELECTRIC  CELLS  135 


in  the  focal  plane  of  the  lens  L^.  It  was  mounted  on  the  table  T, 
which  could  be  moved  through  the  spectrum  by  means  of  the 
fine  screw  R.  This  table  was  inside  a heavy  black  box  K,  rigidly 
attached  to  a solid  pier.  The  entire  optical  system  was  made 
purposely  heavy,  so  that  not  even  the  slightest  displacement 
could  take  place  and  hence  change  the  calibrations.  One  can 
readily  see  that  a very  small  change  in  any  part  of  such  a com- 
plicated system  would  throw  the  whole  arrangement  out  of  order. 
The  scale,  S,  on  the  outside  of  the  box  was  rigidly  connected  with 
the  slit  L.  It  was  the  sliding  of  this  scale  past  a fine  reference 
mark  M that  was  calibrated,  first  in  ergs  and  then  in  wave-lengths 
of  light. 

The  mounting  for  the  slit  L was  made  to  exactly  fit  the  mounting 
of  the  thermo-couple  used  in  the  energj-  calibration.  On  the  top 
of  the  table  were  bored  four  holes,  a,  b,  c,  d (Fig.  2).  Into  these 
four  holes  fit  the  legs  of  a light-tight  metal  box,  specially  constructed 
for  containing  the  photo-electric  cell  under  test.  The  shutter  on 
this  box  for  admitting  and  cutting  oil  the  light  to  the  cells  was 
situated  so  that  it  was  directly  behind  the  slit  L.  This  shutter 
was  manipulated  by  the  operator  at  his  observing  position  by  means 
of  pulleys  and  strings.  The  apparatus  was  so  arranged  that  at 
the  same  place  where  these  strings  were  situated  the  operator 
could  adjust  the  resistance  R2,  read  the  ammeter  A , and  note  the 
photo-electric  galvanometer  deflections. 

'I'he  metal  box  container  hafl  two  hard  rubber  insulators  for 
supporting  the  photo-electric  cell  and  two  insulating  plugs  and 
binding  posts  for  the  lead  wires,  d'he  positive  lead  was  made  of 
sulphur,  and  the  negative  one  of  hard  rubber.  A binding  post 
attached  directly  to  the  box  served  the  purpose  of  grounding  the 
box  and  the  outside  of  the  photo-electric  cells. 

2.  I'he  pholo-elcctric  cells.  'Fhere  were  thirty  cells  used  in  the 
investigation,  including  all  of  the  alkali  metals  and  the  colloida 
modification  of  the  alkali  hydrides  of  sodium,  potassium,  rubidium, 
and  cassium.  d’hey  all  contained  argon  at  a low  pressure,  except 
two  cells  where  neon  was  used.  Three  kinds  of  glass — common, 
pyrex,  and  quartz — were  investigated.  .All  of  the  cells  were  made 


136 


ELEANOR  FRANCES  SEILER 


by  Dr.  Jakob  Kunz  in  this  laboratory.  A full  description  of  their 
construction,  with  the  exception  of  lithium,  has  been  given.^ 

The  making  of  the  lithium  cell  was  somewhat  of  a task,  because 
of  the  danger  encountered  in  trying  to  distil  lithium,  and  of  the 
difficulty  in  obtaining  a solute  for  it.  At  the  suggestion  of  Dr. 
A.  G.  Loomis,  of  the  chemistry  department,  we  tried  dissolving  it  in 
aethylamine,  and  were  eventually  successful.  The  necessary 
condition  for  its  solution  is  that  the  aethylamine  be  absolutely  dry 
and  that  it  be  in  the  presence  of  a trace  of  ammonia. 

The  aethylamine  was  made  according  to  the  following  directions, 
the  chemical  reaction  being 

(C.HsNH,+HBr) +NaOH  = NaBr +C,HsNH,+H,0. 

Seventy  grams  of  aethylamine  hydrobromide  were  placed  in  a 
500-cc  pyrex  glass  flask,  fitted  with  a tight  cork  stopper  holding  a 
separatory  funnel  and  a reflux  condenser  kept  at  about  30°  C. 
To  the  top  of  the  reflux  condenser  was  attached  a drying  column 
of  solid  NaOH,  and  this  connected  to  a downward  condenser  of  the 
spiral  type,  cooled  with  ice  and  salt.  The  receiver  was  also  cooled 
with  a mixture  of  crushed  ice  and  salt  and  had  a NaOH  tube 
attached.  All  stoppers  were  of  cork  to  prevent  the  product  from 
becoming  yellow  because  of  attack  of  rubber  by  the  amine. 

About  200  grams  of  a 30  per  cent  solution  of  NaOH  was  slowly 
dropped  into  the  flask  through  the  separatory  funnel,  and  when  all 
had  been  added,  the  mixture  was  first  heated  with  warm  water  and 
later  with  a flame,  refluxing  until  no  more  amine  condensed  into  the 
receiver.  The  crude  amine  was  then  redistilled  three  or  four  times, 
using  metallic  lithium  as  a drying  agent.  The  final  product  was  a 
pure,  colorless,  dry  fluid.  Metallic  lithium  was  put  into  it,  in  a 
vessel  connected  with  a cell.  After  standing  ten  hours  in  the 
aethylamine  in  the  presence  of  a trace  of  ammonia,  the  lithium 
was  dissolved  and  the  clear  liquid  became  a deep-blue  color.  This 
solution  was  poured  into  the  cell  and  the  aethylamine  pumped  out, 
leaving  a uniformly  distributed  surface  of  grayish  blue  lithium. 
The  cell  was  then  partly  filled  with  argon  at  low  pressure  and 
sealed  off. 

^Physical  Review,  7,  62,  1916. 


COLOR-SENSITIVENESS  OF  PHOTO-ELECTRIC  CELLS  137 


3.  The  set-up.  A diagrammatic  sketch  of  the  connections  is 
shown  in  Figure  3.  The  source  of  potential  E was  a set  of  new 
dry  cells.  The  voltage  ranged  anywhere  from  100  to  310  volts, 
depending  upon  the  kind  of  cell  used.  The  potential  was  measured 
by  a Weston  direct-current  voltmeter,  V.  All  of  the  connecting 
wires,  as  well  as  the  battery  itself,  were  insulated  by  parafi&n. 

The  galvanometer,  G,  which  measured  the  photo-electric  current, 
was  a Leeds  and  Xorthrup  high-sensitivity  type.  It  had  a sen- 
sitiveness of  8X  io~'°  amperes  for  a scale-distance  of  1.25  m,  and  a 
critical  damping  resistance  of  9600  ohms.  To  safeguard  against 
short  circuit,  a resistance  of  100,000  ohms  was  inserted  in  the 


circuit.  The  outside  of  the  cell,  the  positive  terminal  of  the  bat- 
tery, and  the  box  B,  Figure  3,  containing  the  photo-electric  cell, 
were  all  earthed.  When  working  with  the  lithium  cell,  a quadrant 
electrometer  had  to  be  used  to  measure  the  photo-electric  current, 
because  of  the  small  light  intensities  in  the  region  of  its  maxi- 
mum sensitivity,  and  the  rather  low  sensitiveness  of  the  cell. 

4.  Method  of  procedure.  After  the  calibrations  were  made, 
great  care  was  taken  that  no  part  of  the  system  was  disturbed, 
d'hen  a characteristic  curve  was  made  of  the  first  cell  to  be  tested, 
after  which  a preliminary  sensitiveness-curve  was  run  and  finally 
the  real  sensitiveness-curve  data  obtained,  d'his  procedure  was 
repeated  for  all  the  cells  used.  'Fhe  process  will  now  be  discussed 
a little  more  in  detail. 


ELEANOR  FRANCES  SEILER 


138 

The  cell  was  mounted  in  the  box  K (Fig.  2),  the  approximate 
point  of  maximum  galvanometer  deflection  was  found  by  referring 
to  the  work  of  T.  Shinomiya,  and  the  cell  placed  at  that  particular 
wave-length.  Starting  with  a low  voltage,  around  80,  until  a 
deflection  of  about  40  mm  was  obtained,  the  cell  was  moved  back 
and  forth  until  the  exact  maximum  was  found  by  trial.  Keeping 
this  position,  the  deflections  for  varying  voltages  were  noted. 
The  upper  limit  of  the  voltage  was  determined  by  the  unsteadiness 
of  the  galvanometer  deflections.  This  unsteadiness  indicated  that 
the  arcing  potential  was  being  reached.  If  the  cell  was  allowed  to 
glow  much,  its  sensitiveness  decreased.  A slight  glowing  often- 
times increased  the  sensitivity.  Glowing  could  easily  be  detected 
by  the  galvanometer  deflections  suddenly  going  off  the  scale. 
Whenever  this  happened,  an  entirely  new  characteristic  had  to  be 
run.  The  Na  and  NaH  cells  were  the  most  difficult  to  handle,  for 
the  characteristic  curve  rises  so  rapidly  that  a variation  of  4 volts 
(one  dry  cell)  often  caused  great  unsteadiness  in  the  deflections, 
and  sometimes  glowing.  Barring  this  unsteadiness,  the  other 
readings  were  very  steady  and  could  readily  be  repeated  at  the  time 
when  the  data  were  taken.  If,  however,  a characteristic  was  run 
shortly  after  a cell  was  made,  and  again  a day  or  two  later,  it  was 
found  to  change  somewhat.  After  a week’s  time  very  little  change 
took  place.  All  the  data  were  taken  on  cells  more  than  a week  old. 
This  variation  was  more  marked  in  the  hydrides  of  the  alkali  metals 
than  in  the  pure  metals. 

Immediately  after  running  a characteristic,  the  curve  was  drawn, 
plotting  volts  against  photo-electric  current.  Some  characteristic 
curves  are  shown  in  Shinomiya’s  paper. 

A constant  potential  difference  was  then  chosen  from  the 
horizontal  portion  of  the  curve,  or  at  most  up  only  a short  distance 
on  the  vertical  part.  In  no  case  was  such  a voltage  used  as  to  make 
the  galvanometer  deflections  unsteady,  which  occurs  when  the 
critical  potential  is  approached. 

Having  selected  a safe  potential,  one  that  would  give  reasonably 
large  deflections  (100  mm  or  more),  the  cell  was  started  at  the 
extreme  violet  end  of  the  spectrum  and  by  steps  of  a centimeter 
was  taken  up  through  the  red  end.  This  was  the  preliminary 


COLOR-SENSITIVEXESS  OF  PHOTO-ELECTRIC  CELLS  139 


run  to  determine  approximately  the  position,  not  of  maximum 
galvanometer  deflection,  but  of  maximum  photo-electric  sensitive- 
ness. This  point  varied  somewhat  from  the  former  and  was  found 
by  dividing  the  deflections  by  the  corresponding  energ}'  at  the  given 
scale-reading,  .\fter  a lapse  of  about  fifteen  minutes,  during  which 
time  the  cell  was  left  in  the  dark,  a second  final  run  was  made, 
again  starting  at  the  violet  end.  If  no  lapse  of  time  is  allowed  to 
take  place  between  the  first  and  second  sets  of  data,  the  later 
readings  will  be  higher,  because  for  a short  time  the  cell  becomes 
more  sensitive  when  it  has  been  under  the  influence  of  light.  Two 
readings,  rarely  differing,  were  taken  at  each  point,  and  this  time 
they  were  taken  only  3 mm  apart  in  the  region  of  maximum  sen- 
sitivity. Thus  the  maximum  point  was  accurately  determined  to 
within  one  nfj.. 

Finally  curves  were  plotted  with  wave-lengths  of  light  e.xpressed 
in  nfjL  as  abscissae  and  specific  photo-electric  sensitiveness  as  ordi- 
nates, the  units  being  o.6iXio~‘‘  coulombs  per  erg. 

III.  EXPERLMEXT.VL  RESULTS 

The  question  as  to  how  the  colloidal  modification  of  the  alkali 
hydrides  affect  the  color-sensitiveness  has  been  studied.  It  was 
found  that  the  results  of  sensitizing  by  hydrogen  were  twofold: 

a)  It  shifts  the  maximum  sensitiveness,  Xmax,  to  longer  wave- 
lengths by  an  amount  depending  upon  the  alkali  used.  As  can  be 
seen  from  'I'able  1 below,  where  X^ax  and  the  applied  voltage  of 
all  the  cells  investigated  are  listed,  Nall  shifted  8 yu/x,  KII  16  /n/x, 
Rbll  8 fjLfi,  and  CsM  gave  only  the  slight  change  in  one  mm- 
Figures  6,  7,  and  8 show  this  shifting  graphically. 

b)  d’he  colloidal  modification  of  the  alkali  hydrides  also  in- 
creases the  .sensitiveness;  that  is,  a greater  jihoto-electric  current 
is  obtained  with  a given  applied  voltage  from  a hydride  cell  than 
from  the  pure  metal.  'I'he  sensitiveness  depends  upon  the  applied 
voltage,  but  on  the  whole  the  results  indicate  a larger  ordinate  for 
the  hydride  cells.  l*ohl  and  I’ringsheim'  suggest  that  this  increased 
sensitiveness  is  due  to  the  greater  ease  with  which  the  electrons 

' Vcrhandltiugen  dcr  deutschen  physikalischen  Gcscllschafl^  13,211,  1911;  15,  179, 
191 1. 


140 


ELEANOR  FRANCES  SEILER 


TABLE  I 


Type  of  Cell 

^max 

Applied 

Voltage 

Li 

405 

246 

Na  I 

419 

149 

Na  2 

419 

116 

Mean  for  Na. . . 

419 

NaH  I 

427 

304 

NaH  2 

426 

182 

NaH  3 

427 

308 

NaH  4 

428 

171 

NaH  5 

429 

140 

Mean  for  NaH . 

427 

NaH  Neon 

407 

285 

K I 

440 

167 

K 2 

440 

172 

Mean  for  K ...  . 

440 

KH  I 

457 

119 

KH  2 

457 

136 

KH3 

454 

140 

Mean  for  KH  . . 

456 

KH  Neon 

438 

127 

Type  of  Cell 

^max 

Applied 

Voltage 

KH  Quartz 

462 

145 

KH  Pyrex  i 

457 

154 

KH  Pyrex  2 

457 

127 

Rb  I 

473 

146 

Rb  2 

473 

123 

Pb3 

473 

136 

Rb  4 

473 

III 

Mean  forRb. . 

473 

RbH  I 

479 

140 

RbH  2 

483 

lOI 

RbH  3 

481 

123 

Mean  for  RbH 

481 

RbH  Quartz 

507 

179 

Cs  I 

540 

131 

Cs  2 

538 

II7 

Mean  for  Cs  . . 

539 

CsH  I 

539 

148 

CsH  2 

541 

135 

Mean  for  CsH. 

540 

TABLE  II 

Calibration  Data  Giving  the  Wave-Lengths  and  Energy  Along  the 
Arbitrary  Scale  5 


Scale-Reading 


Wave- 
Length 
in  fi/a 


Energy 
in  Ergs 


40.0.  .. . 

40.5  

41.0. ... 
41.3,  ..  . 
41.5.  ..  . 

41.7.  . 

42.0. ... 

42.3 

42.5  

42.7.  , 

43.0.  . . 
43-3  - ■ • 

43- S-  ■ • 
43 ■ 7 ■ ■ ■ 

44.0.  . . . 

44- 3 

44- 5 

44.7.. .. 

45- 0 


393  o 
397-3 

401 .0 
403.2 
404.9 

406.6 

409-5 

412.0 

414.6 

416.5 

419.6 

423-3 

425-4 

428.5 

433-0 

438.0 
440.8 
445-0 

451-0 


1-53 
2 .04 

3- 84 

4- 59 

5- 89 

6- 55 
8.68 
10.48 
12.56 
14.67 

17-35 

21 . 22 

23-58 

26.85 

32.70 

41-30 

45-20 

51-75 

61.80 


Scale-Reading 

Wave- 
Length 
in  /iM 

Energy 
in  Ergs 

45-3 

456.7 

73-8 

45-5 

462.0 

84-4 

45-7 

466.7 

94-3 

46.0 

473-1 

109. 1 

46.3 

483-2 

127.3 

46.5 

488.7 

137-9 

46.7 

496.6 

151-5 

47-0 

506.3 

172.0 

47-3 

516.7 

191-9 

47-5 

524-3 

205.7 

47-7 

535  - 2 

221 .0 

48.0 

550-6 

239-0 

48.3 

567-7 

259.2 

48-5 

580.2 

270. 1 

48-7 

593-5 

288.2 

49-0 

616.6 

305.3 

49-5 

660.7 

338.5 

50-0 

718.0 

366.7 

COLOR-SENSITIVENESS  OF  PHOTO-ELECTRIC  CELLS  141 


can  emerge  from  the  tiny  globules  of  metal  which  characterize  the 
colloidal  state. 

The  curv^es  of  color-sensitiveness  have  been  obtained  for  all  the 
pure  alkali  metals  and  for  all  the  hydrides  with  the  exception  of  Li. 
The  cur\'es  resemble  resonance  cur\*es,  and  Pohl  and  Pringsheim 
have  suggested  that  the  selective  effect  is  a molecular  resonance 
phenomenon  in  which  the  electron  follows  the  electric  force.  The 
curves,  however,  are  not  pure  resonant  ones  because  of  lack  of 
perfect  symmetry.  If  a center  line  is  drawn  from  the  peak  of  a 
curve  perpendicular  to  the  wave-length  axis  and  then  about  half- 
way between  the  peak  and  the  axis  another  line  is  drawn  perpen- 
dicular to  this  center  line,  the  distances  measured  along  the  latter 
from  the  center  to  the  two  points  where  it  cuts  the  curve  is  a 
measure  of  the  curve’s  symmetry.  In  the  pure  metal  as  well  as  in 
the  hydride  of  the  sodium  and  potassium  cells,  the  distance  from 
the  central  line  to  the  right  side  of  the  curve  was  greater  than  that 
to  the  left;  Cs  and  CsH,  on  the  other  hand,  showed  dissymmetry 
in  the  opposite  direction.  The  Rb  and  RbH  cells  are  almost 
symmetrical.  A sufficient  number  of  points  are  located  on  each 
curve  so  that  its  exact  nature  is  determined.  Many  of  the  points 
have  been  omitted  from  the  drawings  for  the  sake  of  clearness. 
They  all,  however,  fell  upon  the  smooth  curve.  An  e.xamination 
of  the  curves  and  Table  I clearly  shows  how  consistently  the  cells 
operated.  The  position  of  X^ai  for  individual  cells  of  the  same 
metal  fell  in  almost  exactly  the  same  position. 

Figures  6 and  7 show  how  the  gas  in  the  cells  affects  the  jihoto- 
electric  effect.  A sodium  cell  and  a potassium  cell,  each  filled  with 
neon  instead  of  the  argon  of  the  other  cells,  showed  a decided  shift 
of  the  Xman  toward  shorter  wave-lengths.  The  amount  of  shift 
was  20  fifjL  for  sodium  and  18  fxjj.  for  potassium.  That  there  should 
be  such  a difference  between  the  Xma:t  for  the  cells  filled  with  neon 
and  with  argon,  both  being  inert  gases,  is  a rather  surprising  phe- 
nomenon. It  is  probably  connected  with  the  fact  that  the  maximum 
sensitiveness  of  sodium  and  potassium  in  these  cells  deviates  con- 
siderably from  the  values  obtained  for  pure  metals  without  gas. 
This  question  requires  further  study. 


142 


ELEANOR  FRANCES  SEILER 


Figures  6 and  7 indicate  the  effect  that  the  kind  of  glass  used  in 
the  photo-electric  cell  has  on  the  wave-length  of  maximum  sensitive- 
ness. Most  of  the  cells  were  made  of  common  glass,  but  two  cells 
of  pyrex  (KH  Pyrex  i and  KH  Pyrex  2)  and  two  of  quartz  (KH 
Quartz  and  RbH  Quartz)  were  constructed.  Both  pyrex  cells 

TABLE  III 

Variation  in  All  Alkali  Metals* 


Scale  in 
Centimeters 


40.0. 

41.0. 

41. 5. 

42.0. 

42.3. 

42.5. 
43-0- 

43 - S- 

44- 0. 

44- 3- 

44.5. 

44.7. 

45- 0- 

45.3. 

45-5. 

45.7. 

46.0. 

46.3. 
46.3. 

46.7. 

47-0. 

47-3-. 

47. 5- . 

47-7- 

48.0.  . 
48. 3-. 

48. 5- . 

48.7.. 

49.0. . 

49- S-- 

50- 0-. 


Li 


Elec- 

trical 

Deflec- 

tion 


mm 
14.8 
43-8 
70.7 
loi . 2 


133-3 

166.5 


302.5 

321.7 


307.0 

230.0 


21.3 


Deflec- 

tion 

Energy 


1265.0 

1194.0 
IS74-0 

1527.0 


1300.0 

1256.0 


877.0 

681.5 


476.8 

299.7 


79.8 


ir  .8 


8.0 

■4'^ 


Na  2 


Galva- 

nometer 

Deflec- 

tions 


mm 
1 . 2 

4.8 

8.9 

16. 1 
21.4 

26.1 
36.6 

48.3 
57.0 

60.4 


56.8 

53-3 


51.0 

4^7 


44.8 

41.5 

36.6 


31.0 

"1S.8 


10.7 

7.8 


Specific 

Photo- 

Electric 

Sensi- 

tiveness 


62.2 

99.8 

120.9 

148.1 

163.2 

166.2 

168.5 

163.6 

139.2 

116.9 


87.8 

68.9 


47-S 

35-6 


25-9 

19.3 

14.2 


10.4 

3.6 


3.1 

1.8 


K 2 


Galva- 

nometer 

Deflec- 

tions 


2.0 

3.4 

6.6 


11.9 

21.2 

36.8 

55.8 


78.6 

88.7 

96.7 

107.2 

113.3 
113-3 

111.4 

103.7 

93.9 

81 . 1 

57.9 

33.2 

21 . 2 


6.9 


Specific 

Photo- 

Electric 

Sensi- 

tiveness 


Rb  3 


41.7 

46.2 

60.7 


75.8 

97.6 

124.7 

136.4 


139.0 

137.0 
124.9 

116 . 1 

107.4 

96.0 
81.6 
63.2 

34.4 

42.8 

27.0 

13.9 
8.3 


2.3 


0.6 

0.3 


Galva- 

nometer 

Deflec- 

tions 


33 


123 


59 


17 


Specific 

Photo- 

Electric 

Sensi- 

tiveness 


52 


Cs  2 


Galva- 

nometer 

Deflec- 

tions 


1.8 


3.9 

9-0 


47.4 

69.7 


104.7 
123.9 

145. 0 

166.2 

169.0 

144.2 

117.2 

94.7 

48.2 

II . I 

3.8 


Specific 

Photo- 

Electric 

Sensi- 

tiveness 


16.6 


17.9 

22.0 


27.51 


31.2 

34.7 


40.4 


48.7 

51. 6 

56.4 

60.1 

56.6 

44.5 

34.7 

26.2 

12.6 
2.6 
0.9 


*The  sensitiveness  column  in  each  case  but  Li  is  multiplied  by  lo 
sensitiveness  in  coulombs  per  erg. 


giving  the  specific  photo-electric 


gave  excellent  curves,  whose  Xmax  varied  only  one  /x/x  from  that  of 
KH  cells  made  from  common  glass.  The  two  cells  constructed  of 
quartz,  however,  showed  a definite  shift  in  Xmax  toward  longer 
wave-lengths,  by  an  amount  equal  to  6 /x/x  for  the  KH  quartz  and 
26  mm  for  the  RbH  quartz  cell.  If  this  shift  due  to  the  quartz  was 
caused  by  absorption  of  light  in  the  wall  of  the  cell,  one  should 
expect  it  to  be  in  the  opposite  direction. 


COLOR-SEXSITIVEXESS  OF  PHOTO-ELECTRIC  CELLS  143 


Fig.  4a 


Wave-L  ength  in  jj-  ]x 

Fig.  4/1 


5ca/e  for  L 


144 


ELEANOR  FRANCES  SEILER 


A further  experiment  was  carried  on  for  the  purpose  of  detecting 
a fatigue  effect  in  a K and  a KH  cell.  The  temperature,  intensity  of 
light,  and  applied  voltage  were  kept  constant  and  the  galvanometer 
deflections  observed  at  intervals  of  from  ten  to  twenty-four  hours. 
The  voltage  continuously  applied  to  the  K cell  was  no  and  after 


TABLE  IV 

Variations  in  the  Hydride  Cells* 


Scale  in 
Centi- 
meters 

NaH  I 

KH  I 

RbH  2 

CsH  2 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

mm 

mm 

mm 

mm 

S-8 

22.9 

26.8 

53-0 

32-3 

394-0 

58-7 

2.8 

25-7 

59-0 

449-5 

43.0 

108.3 

500.2 

17-5 

80.6 

7-3 

33-6 

2.3 

10.6 

44.0 

203.0 

496.0 

50.0 

122.0 

23-5 

57-4 

II. 9 

12.0 

IQS -5 

345-9 

38-7 

68.4 

45-0 

183.0 

236-4 

127.3 

164-5 

62.9 

81.2 

13-0 

16.8 

88.6 

45 -S 

161.6 

153 -2 

172.4 

163-4 

97.0 

91.9 

21.0 

19-9 

46.0 

I34-S 

98-3 

190.2 

139 -5 

137-S 

100.8 

35-3 

25.9 

46  s 

107.0 

62-0 

183.6 

107.6 

170.6 

99.0 

57-3 

33-2 

47-0 

82.2 

38-1 

161.9 

75-2 

171 .0 

79-5 

95-3 

44.3 

47.5 

54-5 

21 . 2 

126.4 

49 1 

138-2 

53-7 

147.  I 

57-2 

48.0 

30-3 

10.  I 

85-3 

28.5 

88.9 

29.7 

176.4 

58-9 

46.8 

48.5 

14.0 

41 -5 

45-8 

13.6 

46-3 

13.7 

106.  s 

31-4 

49  ■ 0 

7.0 

1.9 

20.1 

5.3 

20.1 

5-3 

31-6 

8-3 

4Q-5 

4.6 

2.0 

10. 0 

2.4 

7.7 

1.8 

5-6 

1-3 

50.0 

3-3 

0.7 

7.0 

I-S 

3.3 

0.7 

1.7 

0.4 

*The  sensitiveness  column  in  each  case  is  multiplied  by  lo — ”,  giving  the  specific  photo-electric 
sensitiveness  in  coulombs  per  erg. 


five  hundred  and  twenty-five  hours’  exposure  to  strong  light  the 
cell  had  increased  in  sensitiveness  by  quite  a large  amount.  The 
galvanometer  deflections  began  with  176  mm  and  gradually  in- 
creased up  to  300  mm  at  the  end  of  the  e.xposure.  The  KH  cell, 
on  the  other  hand,  under  the  same  conditions,  except  that  the 
applied  voltage  was  36,  showed  an  exceptional  constancy.  Slight 


COLOR-SENSITIVENESS  OF  PHOTO-ELECTRIC  CELLS  145 


fluctuations  occurred  during  the  one  thousand  hours’  exposure; 
the  initial  and  final  galvanometer  deflections  were  194  mm  and  191 
mm,  respectively,  showing  that  the  fatigue  is  negligible. 

These  same  two  cells  were  exposed  for  60  hours  without  con- 
tinuously applying  a voltage.  At  the  times  when  the  photo- 
electric galvanometer  deflections  were  taken,  a voltage  of  100  was 
applied  to  the  potassium  cell  and  of  27  to  the  potassium  hydride 
cell.  All  the  conditions  remained  constant  throughout  the  experi- 


Wai/e-Length  In jijji 


Fig.  s 


ment.  The  results  showed  an  increase  in  sensitiveness  for  both 
cells,  the  initial  and  final  deflections  for  the  potassium  cell  being 
206  mm  and  274  mm,  and  those  for  the  potassium  hydride  cell 
being  196  mm  and  316  mm.  It  seems  strange  that  merely  the 
exposure  to  light,  without  the  passage  of  a jihoto-electric  current, 
should  increase  the  sensitiveness  so  much  more  rapidly  than  when 
the  applied  voltage  caused  a current  to  flow  constantly  through 
the  cells,  both  investigations,  however,  point  to  a negative  con- 
clusion in  regard  to  a fatigue  effect. 


146 


ELEANOR  FRANCES  SEILER 


TABLE  V 

Variations  in  Potassium  Cells* 


Scale  in 
Centimeters 

K 

I 

KH  2 

KH  Quartz 

KH  Pyrex  I 

KH  Neon 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

mm 

mm 

mm 

mm 

nun 

41.0 

3.8 

79-0 

2.5 

$2.0 

1 .0 

20.8 

1.7 

34.7 

5.8 

120.7 

41.5 

42.0 

9.8 

90.  2 

6.0 

5S-2 

2.6 

23 -9 

9.6 

42.2 

16.9 

148.1 

42.5 

IS-2 

96.7 

10. 0 

63.6 

4.2 

26.7 

9.0 

57.4 

27.2 

173.0 

43-0 

23  s 

108.2 

16. S 

76.0 

6.9 

31.8 

151 

69.4 

44.0 

202.6 

43S 

36.1 

122.4 

28.4 

96.2 

II  .8 

39.9 

32.2 

109.3 

70.7 

239.6 

44.0 

53  0 

129.4 

49.0 

119.6 

20.1 

49.0 

53-2 

130.2 

105.0 

256.5 

44-S 

74-3 

131-4 

80.2 

141.8 

33.8 

59-8 

145.3 

256.9 

44-7 

84.1 

129.8 

98.7 

152.4 

42.7 

66.0 

g6.6 

149.0 

161.8 

249-8 

45-0 

98.0 

127.4 

I3I.3 

169.8 

59.8 

77-3 

122.7 

158.3 

187.0 

241.9 

85.8 

4S-S 

124.8 

118.3 

178.0 

168.6 

92.0 

87 . 2 

171.3 

162.3 

213.0 

201.8 

129.5 

109.7 

102.5 

86.8 

46.0 

128.9 

94.4 

194.8 

142.6 

112.8 

82.6 

200.0 

146.4 

204.0 

149.5 

119.7 

7S.2 

117.0 

73.4 

46.5 

108.  s 

62.9 

177.9 

103.0 

117.7 

68.2 

210.5 

122.0 

170.0 

98.5 

41.0 

66.8 

31. 1 

144.8 

■ 67.4 

120.0 

SS.8 

212.0 

98.4 

II8.O 

54.8 

47.5 

27-3 

10.6 

90.6 

32.2 

no. 7 

43.0 

184.0 

71. 5 

65.1 

25-3 

48.0 

10.4 

3-5 

38.3 

12.8 

72.0 

24.0 

126.1 

42.1 

30.0 

10. 0 

48. S 

6.7 

1 .9 

14.0 

4.2 

26.3 

7.7 

36.2 

16.6 

14.8 

4.4 

49.0 

5-0 

1-3 

7-3 

1.9 

6.1 

1 .6 

20.0 

5.2 

7-5 

2.0 

49.5 

3-9 

0.9 

5.2 

I . 2 

1-7 

0.4 

II-S 

2.7 

3.5 

0.9 

50.0 

3.0 

0.7 

4.0 

0.9 

0.8 

0.2 

9.4 

2.1 

1.9 

0.4 

*The  sensitiveness  column  in  each  case  is  multiplied  by  10 — ”,  giving  the  specific  photo-electric  sensi* 
tiveness  in  coulombs  per  erg. 


Wai/e-L  ength  in 


Fig.  6 


COLOR-SENSITIVENESS  OF  PHOTO-ELECTRIC  CELLS  147 
TABLE  VI 

V.\RL\TIONS  IX  SODItm  CELLS* 


Scale  in 
Centi- 
meters 

Na  I 

XaH  4 

NaH  5 

NaH  Neon 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

mm 

1.2 

8.2 
16.8 
29.0 

mm 

1-5 

4.0 

6.5 

78.2 
832 

88.3 

mm 

0.9 

2.9 

4.9 

8.2 

46.3 

60.4 
66.6 
75-4 

.mm 

’2.3 

6.4 

10.2 

15. 2 

17.8 

119.9 

133.3 

138.7 
139.0 

135.7 

170.5 

228.3 

267.1 

42.3 

13-4 

102.2 

42. s 

46.0 

292.6 

13-2 

20.1 

25.6 

29.9 

35-0 

84.0 

92.6 

96.4 

101.4 

104.2 

24.9 

31. S 

36.8 
42.0 

49.9 

114.5 

118.5 

124.7 
124.9 

121.8 

27 . 2 
3X.S 

125.3 

118,5 

43.5 

83.8 

284.0 

43.7 

44.0 

106.0 

238.9 

41.2 

100. 5 

I2S-0 

134-0 

146.9 

157-2 

162.8 

163.0 

148.1 
107 -3 

222.4 

207.1 
189.9 

170.1 

154.2 
120.8 

85.8 

49.8 

62.8 

m.i 

57.1 

64.9 

100.8 

96.7 

48:8 

50.0 

53-9 

86.2 

77.2 
69.7 

44.7 

45.0 

76.3 

98.6 

45.3 

45.5 

91.2 
100.0. 

96.0 

71.8 

34.3 

n.3 

86.4 

73-4 

55-6 

33-4 

13-4 

37-8 

97.2 

Si  .2 

38.0 

42.5 

46.5 

96.0 

79-8 

ss-s 

37.1 

47 

45-2 

29.9 

13-2 

21.0 
II. 7 

4.4 

47.5 

18.2 

9.2 

6.4 

4-5 

30 

6.1 

2.8 

1.7 

1 .0 

0.7 

21  .0 

48.5 

2.6 

0.7 

0.8 

0.2 

I . I 

0.3 

50.0 

r -5 

0.3 

0.3 

0. 1 

•The  sensitiveness  column  in  each  case  is  multiplied  by  lo — ",  giving  the  spccihc  photo-electric 
sensitiveness  in  coulombs  per  erg. 


148 


ELEANOR  FRANCES  SEILER 


TABLE  VII 

Variation  in  Rubidium  and  Caesium  Cells* 


Scale  in 
Centimeters 

Rb  2 

RbH3 

RbH  Q 

Cs  I 

Cs  H I 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

Galva- 

nometer 

Deflec- 

tions 

Specific 

Photo- 

Electric 

Sensi- 

tiveness 

mm 
I .o 

20.8 

mm 

mm 

0-5 

10.7 

mm 

mm 

2.8 

25.7 

5-3 

30.4 

1 . 2 

n .0 

0.7 

6.3 

1 .0 

9.2 

7.8 

35-9 

7.2 

33-1 

2.6 

12.0 

I-S 

6.9 

2.1 

9-7 

22.3 

54-5 

18.8 

45-9 

6.1 

14-9 

3.4 

8.3 

6.9 

16.8 

3S-0 

6r.Q 

29.9 

52-3 

lo-S 

18.6 

SS-S 

72.8 

85.5 

99.6 
119. o 

134.5 
138.3 

138.6 

126.6 

71.8 

78.8 

81.0 

84.4 

87.2 

84.4 

80.1 
73-2 

58.2 
8. . 

50.0 

67.2 

80.9 

97.9 
122 . 1 
145.5 

155.9 

162.0 

157.9 

64.6 
72.8 

76.6 
83.0 
89.5 

91.3 

90.4 

85.4 

73.4 

18.3 

25. 5 
31.8 

41.4 

51.2 

77.2 

91.6 
no. 6 
131.8 
144.0 

146.2 
147.7 

129.3 

23.7 
27.6 

30.1 
35-8 

41.2 

48.5 

53-1 

58.3 
61 . 2 
59-9 

56.8 

53-4 

43-3 

8.6 

11 . 1 

20.1 

26.0 

14-7 

13-9 

31.6 

30.0 

24.8 

18.2 

48.7 

35.7 

41  .0 
51.2 
69.8 
90.0 
108.7 
126.2 
132.  I 
122. 5 
93-8 

23-7 

27.0 

32.5 

37.5 

42.2 

45.6 

44.2 
37-8 

27.7 

71.6 

84.1 

106.0 
126.5 

150.4 

171.5 

171 .0 

147.6 
103.9 

75.8 

29.0 

4.3 

1-3 

41.5 

44- 4 
49-3 

52.6 
590 

62.0 
57.2 

45- 5 

30.7 

21 . 1 

7.7 

1 .0 

0.3 

84.4 

32.. 
8. . 

II7.O 

45. 5 

40.5 

I3-- 

6.. 

63.4 

21 . 2 

17.0 

5.. 

27.7 

8.2 

87.1 

25-7 

6.1 
2 . 1 
0.9 

I . . 
0.6 

0.5 

12.4 

6.0 

4.1 

3-2 

1-4 

0.9 

48.5 

25-9 

8.9 

12.7 

6.1 

2.3 

36.5 

6.8 

2.8 

95-8 

1.6 

0.6 

50.0 

*The  sensitiveness  column  in  each  case  is  multiplied  by  10— giving  the  specific  photo-electric  sensi- 
tiveness in  coulombs  per  erg. 


0^ ■ ■ ■ i 1 ^ -nr- 

410  440  470  500  530  560  590  620  650  680  7/0 

Wai/e-L  ength  in  juju- 


Fig.  8 


COLOR-SENSITIVENESS  OF  PHOTO-ELECTRIC  CELLS  149 


IV.  THEORETIC.VL  CONSIDERATIONS 

The  results  here  obtained  cannot  be  used  directly  for  theoretical 
considerations,  because  of  the  effect  which  absorption  of  light  in  the 
glass  wall  and  the  presence  of  gas  in  the  cell  have  upon  them.  INIore- 
over, these  results  are  due  to  both  the  normal  and  the  selective  effects, 
for  they  have  not  been  separated  in  the  present  study.  Conclu- 
sions which  have  been  drawn  from  previous  results  are  only 
approximately  true.  Certain  equations  to  follow,  are  deduced  from 
theoretical  considerations,  and  when  the  values  from  this  study 
are  substituted  in  them,  the  results  show  a coincidence  of  about 
the  same  degree  of  approximation  as  those  previously  obtained. 
Since  the  curves  resemble  resonance  curves,  it  is  natural 

to  assume  that  the  frequency  = :r—  , at  which  the  maximum 

''max 

sensitiveness  takes  place,  is  connected  with  the  resonance  potential 
and  the  resonance  frequency  of  the  alkali  vapors,  where  the  quan- 
tum relation  IV  = and  = //«,-  holds  for  resonance  and  ioniza- 

tion potentials  respectively.  All  photo-electric  phenomena  seem  to 
be  governed  by  the  quantum  relation,  so  that  one  might  expect 
proportionality  between  hiph  and  hn,  or  //h,-.  Thus  propor- 
tionality may  exist  between  npi,  and  the  resonance  and  ionization 

potentials,  i.e.,  the  ratios  and  may  be  constants.  These 

values  are  calculated  both  from  the  Xmi«  found  in  Pohl  and  Prings- 
heim'  and  from  this  study,  and  are  given  in  Table  VIII. 

A glance  will  show  that  the  latter  values  are  more  nearly 
constant,  in  spite  of  the  fact  that  the  values  of  Xmax  for  Na  and  Li 
are  greatly  at  variance  with  the  former.  In  this  work  they  were 
found  to  be  in  the  visible  spectrum  and  others  have  located  them 
in  the  ultra-violet  region.  'I'he  ionization  and  resonance  potentials 
are  those  determined  by  d’ate  and  Foote  for  alkali  vapors.  The 
greatest  variation  from  constancy  in  these  ratios  is  in  the  case  of 

^ for  Na.  It  seems  i)robable  that  the  value  of  V,  for  Na  is  a 

little  high.  Why  should  it  be  higher  than  that  for  Li,  for  F,  and 
Vr  both  seem  to  decrease  as  the  atomic  weight  increases  with  this 
one  exception  ? 

' Die  lichlelektrischcn  Erschcimingen,  j).  33,  Tabic  IX. 


15° 


ELEANOR  FRANCES  SEILER 


It  was  thought  that  possibly  the  product  of  and  the 
melting-point  temperature  of  the  alkali  metals  (Wien’s  law) 
might  be_^constant.  This  calculation  shown  in  Table  IX  gives  fairly 
good  values  for  the  constant  C. 

The  results  as  graphed  in  Figures  4 and  5 show  clearly  that  as 
the  atomic  weight  of  the  alkali  metal  decreases,  (a)  the  position  of 
Xmax  shifts  toward  the  violet,  (b)  the  sensitiveness  in  general 


TABLE  VIII 


Metal 

Ionization 

Potential 

L- 

Resonance 

Potential 

X 

max 

Frequency 

«ph 

"ph 

”ph 

Li 

5-36 

1 .84 

280 

10.68 

1.99 

5.80 

Na 

S-ii 

2.09 

340 

8.81 

I . 72 

4.21 

K 

432 

1 .60 

43  s 

6.88 

1-59 

4'3° 

Rb 

4. 16 

I-5S 

480 

6.23 

1-50 

4,02 

VALUES  OF  ^ AND  FROM  THIS  INVESTIGATION 

Vr 

Li 

5-36 

1 .84 

40s 

7-41 

1.38 

4-03 

Na 

S-ii 

2.09 

419 

7 . 16 

I .40 

3-43 

K 

4-32 

1 .60 

440 

6.82 

1.58 

4. 26 

Rb 

4. 16 

I-SS 

473 

6.34 

I 52 

4.09 

Cs 

3.88 

I-4S 

539 

S' 56 

1-43 

3 83 

TABLE  IX 


Element 

^max 

T 

^ m 

CXios 

Li 

405 

458.0 

1.86 

Na 

419 

370.5 

1-55 

K 

440 

335-3 

1 .48 

Rb 

473 

311-0 

1-47 

Cs 

539 

299.0 

I .61 

increases,  (c)  the  resonant  point,  if  such  it  may  be  called,  becomes 
more  marked,  that  is,  the  lithium  cuiA^e  is  narrower  than  that  of  Cs. 
In  order  to  show  this  latter  point,  an  extra  Figure  4b  has  been 
made,  where  the  curves  are  so  drawn  that  the  maximum  ordinates 
are  all  equal. 

In  regard  to  the  shifting  of  Xmax  as  we  go  from  Cs  to  Li,  it 
is  known  that  a similar  relation  holds  for  the  “convergence  fre- 
quency” of  the  principal  spectral  series  for  these  alkali  metals. 


COLOR-SENSITIVENESS  OF  PHOTO-ELECTRIC  CELLS  151 


It  may  be  connected  with  the  increase  in  atomic  volume.  The 
colloidal  state  increased  the  volume  and  shifted  X^ax  toward  longer 
wave-lengths,  so  it  is  natural  to  suppose  that  a large  atomic  volume 
means  that  the  electron  is  held  less  firmly  and  consequently  can 
be  set  in  motion  by  a quantum  of  relatively  small  magnitude — that 
is,  at  a longer  wave-length — than  in  the  case  of  a substance  with 
small  atomic  volume.  Therefore  Cs,  having  an  atomic  volume  five 
times  larger  than  Li,  would  have  a Xn,a.x  of  a much  longer  wave- 
length. 

Two  formulas,  i and  2,  have  been  suggested  by  Lindemann' 
which  are  partly  of  an  empirical  character. 

e I 

nph=~-\i  , or 

27T  \mO 

e 'dfh  , . 

nph  = \ 2 \ — ...  (i) 

7T  \niM 


where  nph  is  the  characteristic  violet  frequency  or  the  maximum 
color-sensitiveness  frequency,  e the  charge  on  the  electron  in 
electrostatic  units,  the  valency,  m the  mass  of  the  electron, 
r one-half  the  distance  between  the  centers  of  two  neighboring 


T.XBLE  X 


Element 

<i 

Atomic 

Weight 

grams 

»phX'°~" 

^max 

Calculated 

^max 

Observed 

Li 

o-Si 

6.94 

1 1 . 17 

1 . 276 

235 

405 

Na 

0.97 

23 .00 

37.09 

0.991 

303 

419 

K 

0.86 

39-10 

63.02 

0.771 

390 

440 

Kb 

I-S2 

85-45 

137.60 

0.642 

467 

473 

Cs 

1.88 

132.81 

214.20 

0.574 

523 

539 

atoms,  d the  density,  and  M the  atomic  mass.  Table  X shows  the 
comparison  between  the  observed  X^a*  and  those  calculated  from 
Formula  r. 

The  second  formula  is  a combination  of  Haber’s  unexplained 
equation  nph  = M jm,  where  is  the  characteristic  red  frequency 

' Bcrichle  di’Hlschen  physikalischen  Gesellschajl,  13,  1107,  1911. 


152 


ELEANOR  FRANCES  SEILER 


emitted  by  the  vibrating  atom,  and  Lindemann’s  melting-point 
equation; 


n,r  = 


V 2,k 


27T 


3~\1  \mV^ 

47r/ 


Substituting  the  latter  in  the  former,  we  get 


nph  — 


f/v 


(2) 


where  K is  a constant  and  V is  atomic  volume.  A calculation  of 
Uph  from  Formula  2 shows  that  the  agreement  between  theory  and 
experiment  is  unsatisfactory. 

V.  SUMMARY  AND  CONCLUSIONS 

Experimental  results  show: 

1.  That  due  to  the  formation  of  a hydride  there  is  a shifting 
of  the  point  of  maximum  color-sensitiveness  (Xmax)  toward  longer 
wave-lengths  for  all  the  alkali  metals. 

2.  That  due  to  an  increase  in  the  atomic  volume  Xmax  shifts 
toward  longer  wave-lengths  as  we  go  from  Li  to  Cs. 

3.  That  the  curves  of  color-sensitiveness  become  broader  with 
increase  in  atomic  weight. 

4.  The  curves  of  color-sensitiveness  for  the  entire  alkali  group 
have  been  completed. 

5.  That  the  effect  of  making  photo-electric  cells  of  quartz  is  to 
cause  the  position  of  Xmax  to  shift  toward  longer  wave-lengths. 
This  is  contrary  to  what  one  would  expect. 

6.  That  the  presence  of  neon  instead  of  argon  in  the  cells  of  Na 
and  K shifts  Xmax  toward  shorter  wave-lengths. 

7.  That  no  fatigue  effect  can  be  detected. 

Theoretical  results  show: 

I.  The  approximate  constancy  of  the  product  of  the  wave- 
length of  maximum  sensitiveness  and  the  ionization  potential  for 
both  the  pure  metals  and  the  hydrides. 


COLOR-SENSITIVENESS  OF  PHOTO-ELECTRIC  CELLS  153 


2.  The  approximate  constancy  of  the  product  of  the  wave- 
length of  maximum  sensitiveness  and  the  melting  temperatures. 

3.  A fair  agreement  between  observed  values  of  X^ax  and  those 
calculated  from  Formula  i of  Lindemann. 

4.  That  the  agreement  between  theory  and  experiment  in 
Formula  2 of  Haber  and  Lindemann  is  unsatisfactory. 

The  writer  desires  to  express  her  appreciation  to  Professor 
A.  P.  Carman  for  the  use  of  the  laboratory  facilities,  and  to  Dr. 
Jakob  Kunz  for  his  continued  interest  and  suggestions. 

University  of  Illinois 
Urbana 
June  1920 


VITA 


Eleanor  Frances  Seiler  completed  her  secondary  training  at  the 
Cathedral  High  School  of  Denver,  Colorado.  In  1910  she  entered  the 
University  of  Denver  and  from  it,  in  T9r3,  received  her  A.B.  degree, 
and  in  1914  her  A.M.  degree. 

She  has  held  the  following  positions:  assistant  in  physics.  Univer- 
sity of  Denver,  i9ir-i4;  teacher  of  physics  and  mathematics  in  the 
Brighton  (Colorado)  High  School,  1914-15;  scholar  in  physics  at  the 
University  of  Illinois  in  1915-16,  receiving  from  it  the  degree  of  A.M.  in 
1916;  instructor  in  physics  and  mathematics,  College  of  St.  Teresa, 
iprh-ry;  assistant  in  physics.  University  of  Illinois,  I9i7-r9;  fellow  in 
physics.  University  of  Illinois,  1919-20. 


iU 


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